All organisms absorb necessary substances from their surroundings. They synthesize necessary substances using the absorbed substances and also break them down to obtain required energy. They also excrete byproducts or wastes produced during the process. All these processes required by the organisms for maintenance of life are called metabolism.
Metabolism is usually divided into anabolism and catabolism. Anabolism is a process of synthesizing high-molecular-weight compounds from low-molecular-weight organic or inorganic matters absorbed from the surroundings. Typical examples of anabolism are photosynthesis and protein synthesis. In anabolism, light or chemical energy is required. Catabolism is the reverse process of breaking down high-molecular-weight compounds into low-molecular-weight organic or inorganic matters. Examples of catabolism include cellular respiration and digestion. Through catabolism, organisms acquire the energy necessary for activities. In general, anabolism is accompanied by an energy-absorbing endothermic reaction and catabolism is accompanied by an energy-releasing exothermic reaction. Accordingly, metabolism of substance is accompanied by an energy metabolism.